coatings Article Improvement of the Turbine Blade Surface Phase Structure Recovered by Plasma Spraying Vitalii V. Savinkin 1, Petrica Vizureanu 2,* , Andrei Victor Sandu 2,3,* , Tatyana Yu. Ratushnaya 1, Andrey A. Ivanischev 1 and Andriana Surleva 4 1 Department of Transport and Mechanical Engineering, M. Kozybayev North Kazakhstan State University, 81 Pushkin Street, Petropavlovsk 150000, Kazakhstan; [email protected] (V.V.S.); [email protected] (T.Y.R.); [email protected] (A.A.I.) 2 Faculty of Materials Science and Engineering, Gheorghe Asachi Technical University of Iasi, 41 D. Mangeron St., 700050 Iasi, Romania 3 Romanian Inventors Forum, Str. Sf. Petru Movila 3, 700089 Iasi, Romania 4 Department of Analytical Chemistry, Faculty of Chemical Technology, University of Chemical Technology and Metallurgy, 8 Kliment Ohridski blvd, 1756 Sofia, Bulgaria; [email protected] * Correspondence: [email protected] (P.V.); [email protected] (A.V.S.) Received: 7 November 2019; Accepted: 8 January 2020; Published: 10 January 2020 Abstract: This paper presents the results of research on the construction, technological parameters and criteria that control the process of formation of optimal phase structure of austenitic- and martensitic-class material for steam turbine blades. The hypothesis that the established correlation could increase the quality of blade recovery and its resistance against dynamic and vibrational loads was proved. The efficiency of the developed implantation method for the recovery of steam turbine blades was demonstrated. The optimal technological parameters of the process of laser plasma recovery were established empirically, allowing the development of the system for the fine tuning of the phase composition of austenitic- and martensitic-class steel. Keywords: turbine blades; laser plasma recovery; prosthetics; phase structure; thermal cycling 1. Introduction The development of the energy sector, CHP and hydroelectric power plants form the basis for the industrial and innovative development programs of any country, especially in areas with enough water resources [1,2]. The implementation of energy-efficient technologies based on highly concentrated energy sources should ensure the development of domestic science and innovative technologies, and increase the competitiveness of energy products. The prospect of energy industry development is based on the introduction of innovative technologies that increase the efficiency of its infrastructure. For example, there are more than 500 CHP, HPP and other types of plants in the Commonwealth of Independent States (CIS), including more than 50 in Kazakhstan. It has also been planned to build nuclear power plants in several countries in the East, in Asia, and in Europe. At the same time, the share of wear of turbine units of CHP and HPP in the global energy sector is about 65%, while the dynamics of prices for energy resources, repair and maintenance is progressively increasing. Blade turbines are mainly used as an engine to drive generators in the power industry. During operation, the turbine blades are subjected to intense erosion, as well as mechanical and fatigue wear. Emergency failure of a hydroelectric unit of a CHP or HPP associated with a breakdown of a hydro turbine, warping, breakage, erosion wear of the n-stage blades and fatigue failure [2] may lead to technogenic disasters and heat and power supply breakdowns in whole regions of countries [3]. Coatings 2020, 10, 62; doi:10.3390/coatings10010062 www.mdpi.com/journal/coatings Coatings 2020, 10, 62 2 of 15 Technical accidents at several HPPs and CHPs have shown an acute shortage of innovative technologies with regard to the repair, restoration and quality control of power units. To solve this problem, it is necessary to modernize the restoration process and introduce new approaches in the development of innovative methods of surface modification. The analysis of global manufacturers of restoration equipment units, such as Siemens-Sabaros, Plakart and Costolins, has shown that indicators of high quality of restoration can be reached through the application of known physical processes of plasma substances [4,5]. The shortage of restoration technologies necessitates the replacement of worn blades with new ones, which results in enormous material costs. For example, the cost of a new 27th-stage blade of the T-100/120-130 turbine is about 950 euros, and a set of blades for one disc costs more than 106,400 ¿, while the cost of repair of three stages with 112 blades in each is 79,800 ¿. Thus, the repair of the blades of moderately worn turbines is economically feasible, since it does not exceed 25% of the cost of new ones. Based on the results of timing studies, it has been found that about 10% of new blades received for replacement fail [4,5]. The main difficulty in the technological recovery process of turbine blades of CHP is the process of forming the optimal structural-phase state of the metal, eliminating the occurrence of cold-drawn cracks and providing vibration resistance to dynamic loads [6–8]. At present, significant results have been achieved in studies of plasma recovery of turbine blades of CHP [9–14]. When repairing turbine blades according to the traditional method [10,11], a plate shall be welded to the place of the worn section of the edge. The disadvantage of this method of repair is the complexity of manufacturing and welding the plate, as well as the high level of tensile residual stresses arising after welding the plate and protective pads. Previously, the researchers Godovskaya, G.V., Isanberdin, A.N., Jablonski, F. and Kienzler, R. developed a turbine blade recovery method [11,12] that included removal of damaged material, weld overlay of the recovered area, heat treatment to remove residual stresses, and mechanical treatment of the blade. The disadvantage of the method is the use of heterogeneous materials for weld overlaying (alloys of nickel with iron and cobalt with iron, as well as alloys of nickel, cobalt, and iron), which on the one hand provides a combination of the plasticity (δ5 = 15%) of the weld deposit and the high hardness (78–89 HRC) of its surface, while at the same time leading to the formation of a heterophase layered structure, which reduces corrosion resistance [10]. A method for recovering the working blades of steam turbines proposed by Lappa, V.A., Fedin, I.V., Khromchenko, F.A., Stratfull, R.F. and Spellman, D.L. [13–16] included the removal of blades from the rotor, removal of protective pads, mechanical removal of the damaged edge section, multilayer surfacing of the restored edge section, furnace heat treatment, mechanical treatment of the blade and welding-on of stellite protective plates. A significant disadvantage of this method is the lack of heat treatment after welding of the protective pads, as a result of which the blade material retains the high heterogeneity of the structural-phase composition and a high level of tensile residual stresses. The researchers Gonserovskiy, F.G., Bi, G., Gasser, A., Wangyao, P. and Lothongkum, G. were also not able to reduce the residual stresses and high heterogeneity of the structural-phase composition of the material of the restored blade during overlaying and welding in the zone of thermal influence [17–19]. Tensile residual stresses reduce the fatigue resistance of the material, and the structural-phase heterogeneity of the material reduces its corrosion resistance. Thus, there are scientific gaps in the study of the influence of structural changes in the blade feather material on the formation of fatigue defects [9,16,20]. Therefore, it is an important scientific and technical task to develop an innovative recovery technology that can improve the physical and mechanical properties and provide optimal structural and phase composition of CHP and HPP turbine blades. Coatings 2020, 10, 62 3 of 15 The scientific problem is that it is necessary to establish the relationship between the modes of the technological process of recovery and the optimal physical and mechanical properties of the resulting coating (adhesion, hardness, material strength, coefficient of thermal expansion). Thus, the purpose of the study is to increase the resource durability of the turbine by developing an innovative technological process of plasma phase restoration of (substandard) blade structure. 2. Materials and Methods Currently, the Commonwealth of Independent States (CIS) countries typically use plasma technology for power equipment recovery using plasmotrons, multicomponent powders, and wires [10,11,18–20]. Strict reliability requirements for gas turbine blades regulate heat-resistant and heat-proof nickel and cobalt alloys such as Kh25N13, 12Kh18N9T, Kh18N10T, 15Kh17N12V3F, etc. [21,22]. The structure of an alloy consists of a Y-matrix and a relatively evenly distributed reinforcing fine g/-phase. As a result of long-term operation at high temperatures, various kinds of defects (cracks, potholes, erosion, etc.) appear on the surface of the blades. In addition, the alloy undergoes structural transformations, which are expressed, as a rule, in the coagulation and morphology of the strengthening Y/-phase and carbides in the body and along the grain boundaries. Therefore, the high physical and mechanical properties of the surface to be restored depend not only on the material of the restored element but also on the modes of the technological process of restoration. Due to the physical characteristics of the coating formation process, internal stresses increase in the layer with increasing thickness, in turn impeding the process of adhesion with the underlying surface. The low ductility of self-fluxing alloys and the high coefficient of linear expansion create a risk of cracking of the coating during cooling. Therefore, it is necessary to carefully choose the recovery and thermal processing modes [18]. To substantiate the optimal modes of CHP turbine blade plasma recovery, a multifactor experimental method was applied, wherein y1, as the hardness of the sputtered layer, y2, as the tension, and y3 , as the adhesion, characterizing the strength and depth of adhesion, were accepted as − criteria for the optimization of plasma sputtering.